The present invention relates to a power source for an implantable medical device. More particularly, it relates to a dual cell power source for optimizing implantable medical device performance.
A variety of different implantable medical devices (IMD) are available for therapeutic stimulation of the heart and are well known in the art. For example, implantable cardioverter-defibrillators (ICDs) are used to treat patients suffering from ventricular fibrillation, a chaotic heart rhythm that can quickly result in death if not corrected. In operation, the ICD continuously monitors the electrical activity of a patient""s heart, detects ventricular fibrillation, and in response to that detection, delivers appropriate shocks to restore normal heart rhythm. Similarly, an automatic implantable defibrillator (AID) is available for therapeutic stimulation of the heart. In operation, an AID device detects ventricular fibrillation and delivers a non-synchronous high-voltage pulse to the heart through widely spaced electrodes located outside of the heart, thus mimicking transthoratic defibrillation. Yet another example of a prior art cardioverter includes the pacemaker/cardioverter/defibrillator (PCD) disclosed, for example, in U.S. Pat. No. 4,375,817 to Engle, et al. This device detects the onset of tachyarrhythmia and includes means to monitor or detect progression of the tachyarrhythmia so that progressively greater energy levels may be applied to the heart to interrupt a ventricular tachycaria or fibrillation. Numerous other, similar implantable medical devices, for example a programmable pacemaker, are further available.
Regardless of the exact construction and use, each of the above-described IMDs are generally comprised of three primary components: a low-power control circuit, a high-power output circuit, and a power source. The control circuit monitors and determines various operating characteristics, such as, for example, rate, synchronization, pulse width and output voltage of heart stimulating pulses, as well as diagnostic functions such as monitoring the heart. Conversely, the high-power output circuit generates electrical stimulating pulses to be applied to the heart via one or more leads in response to signals from the control circuit.
The power source xe2x80x9cpowersxe2x80x9d both the low-power control circuit and the high-power output circuit. As a point of reference, the power source is typically required to provide 10-20 microamps to the control circuit and a high power pulse to the output circuit. Depending upon the particular IMD application, the high-power output circuit may require a stimulation energy of as little as 0.1 Joules for pacemakers to as much as 40 Joules for implantable defibrillators. In addition to providing a sufficient stimulation energy, the power source must possess a low self-discharge to have a useful life of many years, must be highly reliable, and must be able to supply energy from a minimum packaged volume.
Suitable power sources or batteries for IMD""s are virtually always electrochemical in nature, commonly referred to as an electrochemical cell. Acceptable electrochemical cells for IMDs typically include a case surrounding an anode, a separator, a cathode and an electrolyte. The anode material is typically a lithium metal or, for rechargeable cells, a lithium ion containing body. Lithium batteries are generally regarded as acceptable power sources due in part to their high energy density and low self-discharge characteristics relative to other types of batteries. The cathode material is typically metal-based, such as silver vanadium oxide (SVO), manganese dioxide, etc.
In some cases, the power requirements of the output circuit are higher than the battery can deliver. Thus, it is common in the prior art to accumulate and store the stimulating pulse energy in an output energy storage device at some point prior to the delivery of a stimulating pulse, such as with an output capacitor. When the control circuit indicates to the output circuit that a stimulating pulse is to be delivered, the output circuitry causes the energy stored in the output capacitor to be applied to the cardiac tissue via the implanted leads. Prior to delivery of a subsequent stimulating pulse, the output capacitor must be recharged. The time required for the power source to recharge the output capacitor is referred to as xe2x80x9ccharge timexe2x80x9d.
Regardless of whether an output capacitor(s) is employed, one perceived drawback of prior therapeutic pulsing IMDs is that they often have to be replaced before their battery depletion levels have reached a maximum. When an IMD""s output capacitor is being recharged, there is a drop in battery voltage due to the charging current flowing through an inherent battery impedance. Although this voltage drop may not be significant when the battery is new or fresh, it may increase substantially as the battery ages or is approaching depletion, such that during a capacitor recharging operation, the voltage supply to the control circuit may drop below a minimum allowable level. This temporary drop can cause the control circuit to malfunction. The EMD must be removed and replaced before any such malfunctions occur, even though the battery may still have sufficient capacity to stimulate the heart. Simply stated, the rate capability of currently available lithium-based cells is highly dependent upon time or depth-of-discharge as the cell develops high internal resistance over time and/or with repeated use. For IMD applications, this time or depth-of-discharge dependence limits the battery""s useful life.
One solution to the above-described issue is to provide two batteries, one for charging the output circuit or capacitor and a separate battery for powering the control circuit. Unfortunately, the relative amounts of energy required of the device for the control and charging/output circuitry will vary from patient to patient. The capacity of the battery to power the control circuit can only be optimized with regard to one patient profile. For all other patients, one battery will deplete before the other, leaving wasted energy in the device. An example of such a system is disclosed in U.S. Pat. No. 5,614,331 to Takeuchi et al.
An additional, related concern associated with IMD power sources relates to overall size constraints. In particular, in order to provide an appropriate power level for a relatively long time period (on the order of 4-7 years), the power source associated with the high-power output circuitry must have a certain electrode surface area to achieve the high-rate capability. Due to safety and fabrication constraints, the requisite electrode surface area can only be achieved with an increased cell volume. The resulting cell will satisfy output circuitry power requirements, but unfortunately will be volumetrically inefficient. Even further, recent IMD designs require the power source to assume a shape other than rectangular, such as a xe2x80x9cDxe2x80x9d or half xe2x80x9cDxe2x80x9d contours, further contributing to volumetric inefficiencies. In general terms, then, currently available electrochemical cell designs, especially Li/SVO constructions, will satisfy, at least initially, power requirements for the output circuitry. The inherent volumetric inefficiencies of these cells, however, dictates an end-of-life point at which less than the cell""s useful capacity has been used. Once again, currently available cells exhibit an output circuitry charge time that is highly dependent upon time of use or depth-of-discharge. Over time, the cell""s impedance increases, thereby increasing the resulting charge time. Virtually all IMDs have a maximum allowable charge time for the output circuitry. Once the cell""s charge time exceeds the maximum allowable charge time, the IMD must be replaced. The volumetrically inefficient cell will quickly reach this maximum charge time, even though a large portion of the cell""s capacity remains unused (on the order of 40% of the useful capacity). Thus, regardless of whether the power source incorporates one or two cells, the resulting configuration is highly inefficient in terms of the high-rate battery""s useful capacity.
Manufacturers continue to improve upon IMD construction and size characteristics. To this end, currently available power source designs are less than optimal. Therefore, a need exists for an IMD power source having superior space-volumetric efficiencies and a higher energy density, without a proportional increase in charge time.
One aspect of the present invention provides an implantable medical device including a hermetic enclosure, a low-power control circuit, a high-power output circuit, and a power source and associated circuitry. The low-power control circuitry is located within the enclosure. The high-power output circuit is similarly located within the enclosure and is provided to deliver an electrical pulse therapy. Finally, the power source and associated circuitry is located within the enclosure for powering both the low-power control circuit and the high-power output circuit. The power source and associated circuitry includes a first, high-rate cell, a second, lower-rate cell and a switching circuit. The second, lower-rate cell has a rate capability less than that of the first, high-rate cell, such as with a medium-rate cell or a low-rate cell. The first, high-rate cell and the second cell are electrically connected in parallel to the low-power control circuit and the high-power output circuit. Finally, the switching circuit is electrically connected between the first, high-rate cell and the low-power control circuit. The switching circuit selectively uncouples the first, high-rate cell from the low-power control circuit upon activation of the high-power output circuit. During use, both the first, high-rate cell and the second cell operate in parallel during normal device operation. During operation of the high-power output circuitry, such as a transient high power pulse, the switching circuit disconnects the first, high-rate cell from the low-power control circuit. Requisite power is continuously supplied to the low-power control circuit during the high power pulse by the lower-rate cell.
Another aspect of the present invention relates to an implantable medical device including a hermetic enclosure, a low-power control circuit, a high-power output circuit and a power source. The low-power control circuit and the high-power output circuit are located within the enclosure. The high-power output circuit is provided to deliver an electrical pulse therapy. The power source is similarly located within the enclosure and is provided to power the low-power control circuit and the high-power output circuit. With this in mind, the power source includes a case, first anode and cathode bodies, second anode and cathode bodies and an electrolyte. The first anode and cathode bodies, as well as the second anode and cathode bodies, are disposed within the case. The electrolyte is similarly contained within the case. With this configuration, the first anode and cathode bodies combine to effectively form a first cell, and the second anode and cathode bodies combine to effectively form a second cell, with the first and second cells being activated by a common electrolyte. Finally, the first and second cells are electrically connected in parallel to the low-power control circuit and the high-power output circuit. In one preferred embodiment, the first cell is a high-rate cell, whereas the second cell is a lower-rate cell. Regardless, during use, the first and second cells operate in parallel to power the low-power control circuit and the high-power output circuit. During operation of the high-power output circuit, the first cell provides requisite power to the high-power output circuit, whereas the second cell powers the low-power control circuit. In one preferred embodiment, the second cell acts to at least partially re-charge the first cell.
Yet another aspect of the present invention relates to an implantable medical device including a hermetic enclosure, a low-power control circuit, a high-power output circuit, and a power source. The low-power control circuit and the high-power output circuit are located within the enclosure, with the high-power output circuit configured to deliver an electrical pulse therapy. The power source is similarly located within the enclosure and powers the low-power control circuit and the high-power output circuit. The power source includes a first, high-rate cell and a second, lower-rate cell having a rate capability less than a rate capability of the first, high-rate cell. More particularly, the first, high-rate cell includes an anode, a cathode and an electrolyte. The first, high-rate cell is characterized by a rate capability exhibiting minimal dependence on time up to a preselected, voltage-based elective replacement indicator (ERI) at which at least 50% of the cathode is consumed. In one preferred embodiment, the high-rate cell is a lithium-limited cell characterized by a rate capability exhibiting minimal dependence upon depth-of-discharge up to the ERI. During use, the high-rate cell powers the high power control circuit. Because high-rate cell performance is essentially independent of time, a substantially uniform charge time for the high-power output circuit is achieved, while using a majority of the high-rate cell""s capacity over the useful life of the device. Further, the lower-rate cell insures consistent low-power control circuit operation.